My casting for The Morrigan, Queen Mab, Persephone, and Hades.
Saraya-Jade Bevis (Paige) as Morrigan
Morrigan is the Irish goddess of battle and sovereignty, who sometimes takes the form of a crow. Her original Irish name translates as Phantom Queen. She is a certified bad-ass, and though there is no clear link in the mythology, her role sometimes involves foretelling a warrior's death, leading to a possible link with the banshee. And honestly, with a war cry like hers, Paige is the obvious choice.
Katarzyna Dolinska as Queen Mab
Mab is the queen of the fairies and is considered to be the source of dreams. However, in popular culture she is usually portrayed as an antagonist at best and a flat-out villain at worst. I imagine Queen Mab appearing in Storybrooke and kicking the Blue Fairy's ass for usurping power. That would be amazing.
Christina Cole as Persephone
Persephone is the Greek Queen of the Underworld, having been tricked into marriage and ruling by Hades. She is seen as the personification of vegetation, as she was thought to live in the Underworld only during the winter months.
Michael Fassbender as Hades
Because I was having Hex nostalgia and really just wanted him to act opposite Christina Cole again. Hades is the Greek God-King of the Underworld and consort to Persephone. Despite modern connotations of death as evil, Hades was actually more altruistically inclined in mythology. Hades was often portrayed as passive rather than evil; his role was often maintaining relative balance (which I why I don't buy that he abducted Persephone. At all).
Christina Cole & Michael Fassbender as Persephone & Hades
Because. Just... Because.